In order to test drugs in patients with tauopathies, knowledge of the natural history of the frontotemporal lobar dementias (FTLD) must be expressed in terms suitable for designing clinical trials. Without valid estimates of change over 6 or 12 months, instruments appropriate for trials and sample sizes for the trials simply cannot be determined. We are proposing a 4 year, multi-site, longitudinal study of patients with FTLD in which we will recruit subjects using standardized criteria. The specific aims of this project are 1) determine the ratio between change and variance in cognitive, behavioral and functional instruments in order to estimate power to detect treatment effects with each instrument in future clinical trials; 2) perform serial MR imaging to determine the magnitude of change and its variance in global and regional frontal or temporal brain volume in order to estimate power to detect treatment effects on brain volume in future clinical trials; 3) develop a composite FTLD-subtype-specific cognitive instrument for use in clinical trials; and 4) determine whether ApoE genotype and tau haplotype are associated with rate of progression on cognitive, behavioral, functional or imaging in FTLD. We propose to involve 3 Alzheimer Disease Centers (Mayo Rochester/Jacksonville, UCLA and Arizona) plus the University of California- San Francisco to recruit 120 patients with FTLD. We shall recruit patients with the behavioral-dysexecutive syndrome of fronto-temporal dementia, patients with semantic dementia, and patients with progressive nonfluent aphasia. Operational criteria for these 3 syndromes have been developed that will meet rigorous standards suitable for clinical trial recruitment. Subjects will be examined with cognitive, behavioral, functional assessments as well as MR imaging at baseline and 12 months. Key outcomes will include estimates of change and its variability over 1 year on MR imaging, change on cognitive tasks including the composite task, and change on functional and behavioral measures. While the study will also develop a wealth of new knowledge about the relationships between cognition, behavior and brain structure in FTLD, the essential product of this study will be the principles underlying a rationale design for trials of drugs for FTLD.